A WIND-TUNNEL INVESTIGATION OF THE LIMITATIONS OF THERMISTOR ANEMOMETRY

Abstract

An experimental investigation of the characteristics of thermistors, when used as anemometers, has been made. The various modes of operating thermistor micro-anemometers are studied, and the sensitivity and errors discussed. A variable-temperature, variable-speed wind tunnel was constructed for this investigation, to determine the power dissipated in the thermistor as a function of wind speed, wind direction, and air temperature. The thermal lag of the thermistor to environmental changes was studied. It was concluded that operating the thermistor at constant resistance, by means of a specially designed bridge, showed the greatest promise in that this mode of operation was the most sensitive to wind-speed changes, the least sensitive to ambient-temperature changes, and eliminated lag to a great extent. Operated at constant resistance, the lag time of the response of the thermistor to a ?step-function? air-speed change was about 0.085 sec, the error in air-speed indication due to a degree change in ambient temperature was approximately 0.5 per cent on the average, and for direction changes up to 20 per cent. However, much work, particularly on the elimination of temperature and directional errors, must still be done to make thermistor anemometers practical.